The invention concerns an apparatus for paint or lacquer coating of a quasi-endless surface element, capable of coiling, and an appropriate process.
Quasi-endless coated metal sheets, in particular steel and aluminium sheets, are processed in huge numbers in the automobile industry and also for the manufacture of domestic appliances (so-called “white goods”) and furthermore for the production of aircraft and water vessels and of construction panelling. Steel sheets are primarily metallically bare, galvanised or nickel-plated sheets with thickness values in the range between a few tenths of a millimeter and more than one millimeter. As pre-treatment for later painting or plastic coating, a corrosion protection coating and/or a primer coat is already applied to such sheets on the coil.
Such coatings are mostly applied in the liquid state, but occasionally also as powder coatings and, after application, must be rationally dried or cross-linked to arrive at a solid coating. To this end, and as is known, sheets are inductively heated to a temperature around or above 250° C., thus also heating up the coating to over 200° C., therefore drying or cross-linking it.
This process involves very large amounts of energy because it is not primarily the actual processed item, namely the applied coating, that is heated, but the sheet itself, which does not require any heating. By contrast, in certain cases heating of the sheet is even disadvantageous because it is capable of modifying specific physical properties of the sheet that have been adjusted in prior steps through conscientiously coordinated thermal processes, up to here. Incidentally, in view of the very large numbers of tons of coated sheet dried in this way, the energy lost as a result of unnecessary warming of the sheet adds up to considerable amounts in terms of the national economy.
A further considerable disadvantage of the known process consists of the high realization costs of inductive heating lines. In total, the aforementioned disadvantages lead to a price for sheets processed by the so-called “coil coating” method that is still relatively high today, which is manifesting itself increasingly disadvantageously in view of the increasing cost pressure on suppliers in the automobile and consumer goods industries.
The known coil coating processes supply high-quality products with a high throughput per unit of time. It has been found, however, that certain surface defects such as fine cracks and slight flaws can occur when recoiling the coated sheet with the hardened coating which, in certain circumstances, can severely impair usefulness during later use. During further processing, segments of strip with such flaws can results in products of impaired quality which, in certain circumstances, even has to be scrapped.
At an earlier time, the applicant had therefore proposed to realize the drying or cross-linking step in a so-called radiation drier by irradiating the coated surface of the surface element with electromagnetic radiation of a high power density in the range of near infrared, which has its essential active components in the wavelength range between 0.8 and 1.5 μm; cf. DE 101 06 890 A1. A similar process and suitable powder slurries have become known from DE 100 27 444 A1. In view of their considerable advantages, such coil coating processes using NIR® driers have in the meantime experienced widespread use in large industrial steel treatment.
One basic disadvantage of the known large industrial coil coating processes—with inductive or radiation drying—consists of the fact that process control and plant structure are tailored to large throughput volumes. Correspondingly powerful installations are space-consuming and costly and operate efficiently only if considerable use is made of their capacities. With these processes and plants, small orders at a customer's special request cannot be realized rationally because large volumes of scrap sheets are produced during the start-up phase and until constant process control is achieved and, in turn, when shutting down a plant. Due to the small batch size, that increases product costs, which are high anyway.